1. Field of the Invention
The present invention relates to a sheet stacking device containing sheets (recording media) to be supplied to an image forming apparatus, and a sheet feeding device and an image forming apparatus each provided with the sheet stacking device. More particularly, the present invention relates to a configuration for separating sheets tightly contacting each other for feeding.
2. Description of the Related Art
A conventional image forming apparatus (e.g., a copier, printer or the like) is provided with a sheet feeding device with which sheets contained in a sheet stacking device (e.g., a sheet cassette, sheet feeding deck or the like) are sequentially fed from the top of the stack of the sheets by a feed roller and separated by a sheet separating portion one by one to be fed to an image forming portion.
In such a sheet feeding device, cut sheets are used for continuous feed of the sheets. Such cut sheets are often limited to quality paper or plain paper designated by a copier manufacturer.
A variety of sheet separating systems have conventionally been employed to reliably separate sheets one by one for feeding. One such sheet separating system is a separating pad system in which a friction member contacts with a feed roller at a prescribed pressure, for example, so as to prevent double feed of the sheets disposed therebetween.
Another sheet separating system is a retard roller separating system having a pickup roller for sending out the sheets, a feed roller rotating in a sheet conveying direction, and a separating roller driven at a prescribed torque in a direction opposite to the sheet conveying direction and contacting with the feed roller at a prescribed pressure. With this sheet separating system, if a plurality of sheets are sent out by the pickup roller, only the top sheet is allowed to pass through the feed roller and the separating roller, while the sheet beneath the top sheet sent out together is returned to thereby prevent double feed of the sheets.
In order to ensure separate feeding of sheets in these systems, in the case of the separating pad system, for example, the friction force (between the feed roller and the sheet, between the sheets, and between the sheet and the separating pad) and the pressing force (feed roller contact pressure, separating pad contact pressure) are taken into consideration and optimized to surely separate the sheets one by one.
Meanwhile, with diversification of the sheets (recording media) in recent years, there is an increasing demand for image formation, not only on extra thick paper, OHP sheet, art film and others, but also on coated sheets having the surface coated to achieve higher degree of whiteness or gloss in response to the needs of the market for colorization.
In feeding the extra thick paper, however, its own weight may become conveyance resistant, making it difficult to pick up the sheet and thereby cause jamming. Further, in the case of the sheets made of a resin material that tends to be charged, such as the OHP sheet or the art film, when the sheet feeding operation proceeds in low-humidity environments, the sheet surfaces may gradually be charged as the sheets are rubbed with each other, in which case the sheets would tightly contact each other by the Coulomb force, leading to failure in picking up the sheet or occurrence of double feed.
Further, in the case of the coated paper having the sheet surface applied with a coating material such as paint, the sheets tend to stick together particularly when they are stacked in high-humidity environments, which again may lead to failure in picking up the sheet or occurrence of double feed.
Such problem occurs for the following reasons. In the case of the special sheets as described above, although the friction force between the sheets in itself may be equivalent to or less than in the case of the plain paper or the like, a sticking force caused by frictional electrification in low-humidity environments in the case of the resin material sheets or a sticking force generated in high-humidity environments in the case of the coated sheets, would lead to the sticking force between the sheets being much stronger than the friction force therebetween. Thus, the sheets cannot be separated reliably with conventional sheet separating systems.
That is, in the case of the conventional sheet separating systems, the friction force between the sheets alone is taken into consideration. Thus, it would not be possible to surely separate the sheets from each other when the sticking force is applied in addition to the friction force.
In order to cancel such very strong sticking force between the sheets, a sheet separating system for reliably separating the sheets one by one by fanning the sheets in advance by blasting the air onto the side face of the sheet stack to cancel the sticking between the sheets, and then picking up the sheets sequentially from the top of the sheet stack and reliably separating them one by one at a separating portion provided downstream, as disclosed in Japanese Patent Laid-Open No. 11-005643 (corresponding to U.S. Pat. No. 6,015,144), is employed in the print industry and in some copiers.
In the sheet separating system provided with such an air blasting mechanism blasting air onto the side face of the sheet stack, even in the case of the sheets (recording media) highly sticky as described above, the sheets are fanned prior to the feeding to cancel the sticking, so that the separating performance is improved as compared to the above-described systems utilizing only the friction force.
A sheet feeding device provided with such an air blasting mechanism will now be described with reference to FIGS. 9 and 10. FIG. 9 is a cross sectional view of an image forming apparatus, and FIG. 10 is a top plan view of a sheet feeding device 55. The sheet feeding device 55 is arranged beneath an image forming portion (including an image writing unit 31, an image bearing member 35, a charging unit 37, a developing unit 39, a transfer roller 43, and a fixing unit 45) of an image forming apparatus 21. The sheet fed from the sheet feeding device 55 is discharged to a discharge tray 49 after an image is formed thereon at the image forming portion.
As shown in FIG. 9, sheet feeding device 55 includes a sheet cassette 59 for containing sheets S thereon, and a sheet feeding portion 60 for feeding sheets S from sheet cassette 59. Further, as shown in FIG. 10, an end guide 65 is provided on the opposite side of sheet cassette 59 from sheet feeding portion 60 (on the rear end side of the sheets) to regulate the end face of sheets S, stacked in sheet cassette 59, on the opposite side from the sheet feeding direction. To regulate the side face positions of sheets S, a fixed side face guide 67 and a movable side face guide 69 opposite to the guide 67 are provided on the respective sides of sheet cassettes 59. An air blasting mechanism 71 for blasting the air onto the side faces of the stacked sheets is provided on the same side as the fixed side face guide 67. The air blasting mechanism 71 has a fan or the like, so that the air is blasted onto the side ends of the sheets from an air blow-off port 73 as appropriate.
The air blasting mechanism 71 operates to blast the air onto the side faces of the sheets from the air blow-off port 73 of the air blasting mechanism 71. The sheets are fanned as the air thus blown off lifts up the upper sheets S, to cancel sticking between the sheets. Accordingly, the sheets can reliably be separated one by one to be fed.
In the conventional sheet feeding device using such fanning mechanism with the air, however, it is necessary to provide in advance an air blasting mechanism having a fan, duct and others as described above. For a user who does not need to feed special sheets of paper, the device is provided with an unnecessary mechanism, which would disadvantageously increase the cost and size of the device.
Further, provision of the air blasting mechanism in the device main body increases the space thereof, which goes against downsizing of the image forming apparatus that is eagerly longed for from the standpoint of effective use of space (space saving) by small offices and home offices rapidly increasing in number in these years.